In internal combustion engines, in particular in petrol-operated motor vehicle engines, camshafts are used to actuate the “gas exchange valves.” The cams of the camshafts conventionally bear against cam followers, for example bucket tappets, rocker arms or oscillating levers. If a camshaft is transferred into rotation, the cams roll along the cam followers which, in turn, actuate the gas exchange valves. The opening duration and the opening amplitude, but also the opening and closing times of the gas exchange valves are therefore defined by the position and the shape of the cams.
The angular displacement of the camshaft with respect to a crankshaft in order to obtain optimized control times for various rotational speed and load states is referred to as camshaft adjustment. One structural alternative of a camshaft adjuster operates, for example, according to the “pivoting motor principle.” In this case, there is a stator and a rotor which lie coaxially with respect to each other and are movable relative to each other. The stator and the rotor together form hydraulic chambers, which are simply referred to in the following as chambers. One pair of chambers here is in each case delimited by webs of the stator and is divided by a respective vane of the rotor into two mutually opposed chambers, the volumes of which are changed in an opposed manner by a rotational movement of the rotor relative to the stator. In the maximum position of adjustment, the respective vane bears against one of the webs at the edge of the stator.
The relative rotational movement of the rotor takes place by means of adjustment of the vane by a hydraulic medium, for example oil, being introduced into the chambers via radial channels and pushing the vane away. With the adjustment of the rotor, the camshaft, which is fastened to the rotor, is adjusted, for example, in the early direction, i.e. to an earlier opening time of the gas exchange valves. With adjustment of the rotor in the opposite direction, the camshaft is adjusted in the late direction in relation to the crankshaft, i.e. to a later opening time of the gas exchange valves.
The camshaft adjuster is controlled by an electronic control device which, on the basis of electronically recorded characteristic data of the internal combustion engine, for example rotational speed and load, regulates the inflow and outflow of pressure medium to and from the individual chambers via a control valve which is designed, for example, as a proportional valve.
In the event of an insufficient supply of pressure medium, as is the case, for example, during the starting phase of the internal combustion engine or during idling, alternating torques which are transmitted by the camshaft to the rotor lead to the rotor being moved in an uncontrolled manner, which has the consequence that the vanes beat to and fro within the working spaces, which promotes the wear and causes an undesirable production of noise. In addition, the phase position fluctuates greatly between crankshaft and camshaft, and therefore the internal combustion engine does not start or runs noisily. In order to avoid this problem, hydraulic camshaft adjusters are equipped with a locking mechanism for locking stator and rotor for conjoint rotation. A locking mechanism of this type comprises, for example, an axial pin which is accommodated in the rotor, is displaced out of the receptacle thereof in the axial direction by means of a spring and can engage in a form-fitting manner in a locking slotted guide which is formed, in particular, in a sealing cover for the stator and the rotor. For unlocking, the pin is acted upon on the end side by pressure medium and displaced back into the receptacle thereof in the rotor.
The stator and rotor are locked in a camshaft phase position which is referred to as the basic position and is thermodynamically favorable for starting the internal combustion engine. Depending on the specific design of the internal combustion engine, an early position, late position or intermediate position is selected as the basic position. With reference to the driving direction of the stator or camshaft, the late position corresponds to an end rotational position of the rotor in the trailing direction (in which the volumes of the advancing pressure chambers are at maximum), the early position corresponds to an end rotational position of the rotor in the advancing direction (in which the volumes of the trailing chambers are at maximum), and the intermediate position corresponds to a phase position located between the early and the late position. An intermediate position located at least approximately in the middle between the early and the late position is referred to as the middle position. An adjustment of the phase position of the rotor in a rotational direction identical to the driving direction of the stator or camshaft is referred to as early adjustment. An adjustment of the phase position of the rotor in a rotational direction opposed thereto is referred to as late adjustment.
If, when the internal combustion engine is turned off, the basic position is not reached (for example, in the event of the motor stalling), the rotor is automatically adjusted into the late position owing to moments of friction. If the rotor is intended to be locked in the early position or an intermediate position, special measures by means of which the rotor is adjusted relative to the stator therefore have to be taken. For example, torsion springs are provided for this purpose in conventional camshaft adjusters, the torsion springs prestressing the rotor in the direction of the desired basic position. In a concept for the locking of stator and rotor, the latter are already locked during the turning off of the motor. This affords the advantage that, during the subsequent starting of the motor, the motor is started directly in the middle position. However, this is associated with a large outlay on control and monitoring, since the control valve has to be strategically energized in accordance with the current angular position when the motor is turned off and the angular position has to be constantly monitored.
DE 10 2008 011 916, for example, describes a camshaft adjuster with an axial multiple grid-type locking means. However, a disadvantage of this camshaft adjuster is that it functions only if there is no oil pressure in the early chamber.